[otbn] Add start/end symbols for IMEM and DMEM to linker script
[otbn] Add start/end symbols for IMEM and DMEM to linker script

The linker script now has symbols for the start and end of instruction
and data memory, respectively. These symbols can be used to calculate
the size of the application and the data when loading it.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan